JP Patent Publication 2009-293679A discloses a conventional rotation transmission device for selectively transmitting driving force to the front wheels, i.e. auxiliary drive wheels, of a front-engine-rear-drive (FR)-based four-wheel drive vehicle.
The rotation transmission device disclosed in JP Patent Publication 2009-293679A includes a control retainer and a rotary retainer mounted between an outer ring and an inner ring mounted in the outer ring such that crossbars of each retainer are arranged circumferentially alternating with the crossbars of the other retainer, opposed pairs of rollers, each pair being received in a pocket defined between each adjacent pair of crossbars, and elastic members disposed between the respective opposed pairs of rollers and biasing the respective pairs of rollers away from each other to a standby position where the rollers can instantly engage a cylindrical surface formed on the inner periphery of the outer ring and cam surfaces formed on the outer periphery of the inner ring. When the inner ring is rotated in one direction while the rollers are in the standby position, one of each opposed pair of the rollers engages the cylindrical surface and the cam surface, so that the rotation of the inner ring is transmitted to the outer ring.
This device further includes an electromagnetic clutch mounted on an input shaft connected to the inner ring for axially moving the control retainer. When the control retainer is moved by the electromagnetic clutch, the control retainer and the rotary retainer are rotated relative to each other in the direction in which the circumferential widths of the pockets decrease by the action of a torque cam provided between the opposed surfaces of flanges of the control retainer and the rotary retainer. When the retainers are rotated in this direction, the opposed pairs of rollers are pushed by the respective crossbars to a disengaged position. Thus, no rotation is transmitted from the inner ring to the outer ring in this state.
In this rotation transmission device, when the control retainer is moved in the direction in which the flange of the control retainer moves away from the flange of the rotary retainer, the control retainer and the rotary retainer are rotated relative to each other in the direction in which the circumferential widths of the pockets increase under the biasing force of the elastic members disposed between the respective opposed pairs of rollers, so that the rollers instantly engage the cylindrical surface and the cam surfaces. Thus the distance by which each roller moves in the rotational direction until it engages is extremely small, so that the rollers can quickly engage.
In the rotation transmission device disclosed in JP Patent Publication 2009-293679A, since the control retainer and the rotary retainer are entirely received in the outer ring, the outer ring is axially long and thus the entire rotation transmission device is heavyweight.